1. Field
Exemplary embodiments of the disclosure relate to the manufacture of ceramic honeycombs, and more particularly to the heating of cellular ceramic green bodies to remove binding constituents therefrom during firing to reaction sinter the green bodies to ceramic honeycombs.
2. Discussion of the Background
Ceramic honeycombs composed of refractory ceramic materials such as cordierite, silicon carbide, aluminum titanate and the like are widely used for the manufacture of catalytic substrates and particulate filters. Such substrates and filters are presently needed for the removal of pollutants such as carbon monoxide, nitrogen and sulfur oxides, unburned hydrocarbons and particulates such as soot from combustion engine exhaust gases or stack gases from industrial combustion processes.
The firing of cellular ceramic green bodies to convert them to ceramic honeycombs first requires the debindering or removal from the bodies of various organic binding or pore-forming constituents. Those constituents are required in the earlier forming stage of manufacture for the shaping of plastic mixtures of ceramic precursor powders and binding constituents into self-supporting green cellular shapes. Shaping is typically by extrusion of the plastic mixtures through honeycomb extrusion dies.
Significant manufacturing difficulties can arise where the green honeycomb shapes comprise more than about 5% by weight of organic constituents such as cellulosic binders and/or pore forming additives such as starch that are combustible. High rates of cracking can be observed in the fired ware if the removal of organic binding and/or pore-forming constituents is not carefully managed. The debindering of large cellular green bodies, such as those used for the production of cordierite particulate filters for treating heavy duty diesel engine exhaust streams is particularly problematic.
The predominant source of cracking during the debindering of cellular ceramic green bodies is thought to be an uncontrolled burning (thermal runaway) of the organics within the cores of the cellular green ware. Such burning generates large thermal gradients in the green ware that in turn produce thermal stresses great enough to cause cracking during debindering.
A number of approaches to address such cracking have been proposed. These include the use of low oxygen debindering atmospheres to reduce organics combustion rates, the use of reduced heating rates during debindering to reduce internal temperature differentials within the bodies, and the use of increasing levels of gas circulation through the cells of the bodies during debindering in order to improve temperature uniformity within the bodies. Nevertheless, significant levels of cracking in fired ceramic honeycombs are still encountered.